Hydrothermal synthesis of ZnO nanowires and nanobelts on a large scale

Materials Chemistry and Physics

106 (2007) 58–62

Hydrothermal synthesis of ZnO nanowires and nanobelts on a large scale Hanmei Hu a,b,?,Xianhuai Huang c,Chonghai Deng d,Xiangying Chen b,Yitai Qian b

a Department of Material Science and Engineering,Anhui Institute of Architecture and Industry,Hefei,Anhui230022,People’s Republic of China

b Structure Research Laboratory and Department of Chemistry,University of Science and Technology of China,

Hefei,Anhui230026,People’s Republic of China

c Department of Environment Engineering,Anhui Institute of Architecture an

d Industry,Hefei,Anhui230022,People’s Republic of China

d Hefei University,Hefei,Anhui230022,People’s Republic of China

Received28August2006;received in revised form30April2007;accepted6May2007

Abstract

ZnO nanowires(～60%)and nanobelts(～40%)have been fabricated on a large scale via a low temperature one-pot hydrothermal technique. Na2CO3was introduced not only as alkaline source but also as a controllable reagent for the crystal growth of ZnO.The comparison experiment results indicate that the adding amount of Na2CO3greatly affect the length/diameter aspect ratios of1D ZnO nanocrystals.In addition,the introduction of surfactant SDSN was indispensable in controlling the growth of belt-like ZnO.Room temperature photoluminescence spectrum showed a weak UV band emission at379nm and a strong broad yellow emission around564nm.A possible mechanism on the formation of the ZnO nanowires was proposed.

Keywords:Nanostructures;Chemical synthesis;Electron microscopy;Optical properties

1.Introduction

Nanorods and nanowires represent a class of one-dimensional (1D)nanostructures,in which the carrier motion is restricted in two directions such that they usually show interesting proper-ties that differ from those of the bulk or spherical nanoparticles of the same chemical composition[1,2].These nanostructures have potential applications as important components and inter-connects in nanodevices[3,4].

ZnO is one of the most promising materials for optoelectronic applications due to its wide band gap of3.37eV and large exciton binding energy of60meV[5].It has been recognized as one of the promising nanomaterials in a broad range of high-technology applications,such as photodetector[6],light-emitting diode[7], gas sensor[8],solar cell[9],optical modulator waveguide[10], and surface acoustic wave devices[11],etc.

In recent years,various methods have been used to syn-thesis1D ZnO nanostructures,such as thermal evaporation process[12–14],chemical vapor deposition[15,16],metalor-

?Corresponding author at:Department of Material Science and Engineering, Anhui Institute of Architecture and Industry,Hefei,Anhui230022,People’s Republic of China.Tel.:+865513526891.

E-mail address:hmhu@https://www.360docs.net/doc/ce7458596.html,(H.Hu).ganic vapor-phase epitaxy[17],microwave plasma deposition [18],pyrolysis[19],hydrothermal method[20,21],and etc.The hydrothermal method is a promising one for fabricating ideal nanomaterial with special morphology because of the low cost, low temperature,high yield,scalable process.In the present work,ZnO nanowires(～60%)and nanobelts(～40%)have been produced in large quantities,using ZnCl2as zinc source, Na2CO3as mineralizer,and sodium dodecyl sulfonate(SDSN) as morphology controller agent,via a low temperature one-pot hydrothermal technique.

2.Experimental procedure

All chemicals(analytical grade reagents)were purchased from Shanghai Chemical Reagents Co.and used without further puri?cation.In a typical exper-imental procedure,0.2g ZnCl2,1.5g SDSN and20g Na2CO3(～4.72M)were successively added into a50mL Telfon-lined stainless steel autoclave,which was then?lled with distilled water up to90%of the total volume.The obtained reaction mixture was stirred for an additional30min.The autoclave was sealed and maintained at140?C for12h.After the reaction was completed,the result-ing white products were?ltered off,washed with ethanol and hot distilled water for several times,and then?nally dried in a vacuum at60?C for4h.

The phase purity of the as-synthesized products was examined by X-ray diffraction(XRD)using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized Cu K?radiation(λ=1.541874?A). The morphology and size of the obtained ZnO products were further observed by

H.Hu et al./Materials Chemistry and Physics 106 (2007) 58–6259

transmission electron microscope(TEM)and?eld-emission scanning electron microscope(FESEM),which were taken on a Hitachi model H-800(200kV) and a?eld-emission microscope(JEOL-JSM-6700F15kV),respectively.The HRTEM image was taken with a JEOL-2010transmission electron microscope with an accelerating voltage of200kV.SEM images were taken on an X-650 scanning electronic microanalyzer.The photoluminescence(PL)spectrum was recorded on a Steady-state/Lifetime Spectro?uorometer(FluoRoloG-3-TAU).

3.Results and discussions

Fig.1(a)shows the typical XRD pattern of the as-prepared ZnO products.All the re?ections can be indexed to wurtzite structure of ZnO with lattice parameters a=3.247?A and c=5.20?A,in good agreement with the reported data for ZnO (a=3.249?A,c=5.205?A,JCPDS File,5-664).No character-istic peaks were detected for the other impurities such as Zn(OH)2,ZnCO3.Fig.1(b)and(c),respectively,shows the low-magni?cation and high-magni?cation FESEM image of the obtained sample,which indicates that the as-synthesized ZnO products were composed of wire-like(ratio:～60%)and belt-like(ratio:～40%,indicated by the black arrow in Fig.1(c)) nanostructures,and their lengths are up to20?m.The diame-ters of ZnO nanowires are about20–100nm and the widths of ZnO nanobelts are in the range of80–250nm.

Further structural characterizations of the ZnO nanowires/belts were performed by TEM and HRTEM. Fig.2(a)shows the low-magni?cation TEM image of the ZnO nanowires.With increasing the TEM magni?cation,belt-like ZnO nanostructures are apparently observed(Fig.2(b)). Fig.2(c)shows the TEM image of a well-developed single crystal ZnO nanobelt with width of220nm.The SAED pattern of the nanobelt(inserted at the upper left corner of Fig.2(c)) indicates its single crystal nature and its growth direction along c-axis.The typical HRTEM image,recorded from a certain nanowire,is shown in Fig.2(d).The crystal lattice fringes are clearly observed and average distance between the adjacent lattice planes is0.52nm,corresponding to the(0001)plane lattice distance of hexagonal-structured ZnO,which further proves that ZnO nanowires/belts prepared in the present system grow along[0001]direction.

In the present reaction system,the possible formation process for hexagonal ZnO phase under hydrothermal condition can be expressed as follows:

CO32?+H2O→HCO3?+OH?(1) HCO3?+H2O→H2CO3+OH?(2) Zn2++4OH?→Zn(OH)42?(3) Zn(OH)42?→ZnO+H2O+2OH?(4)

A suitable hydrothermal system may be helpful for the nucle-ation and subsequent1D preferential growth of ZnO crystals.It was interestingly found that the amount of mineralizer Na2CO3 play a critical role in the control growth of ZnO nanowires.A series of comparison experiments were performed by chang-ing the adding amount of Na2CO3without using SDSN in the reaction system.Fig.3(a)shows the TEM images of the pre-pared ZnO products when the adding amount of Na2CO3

was Fig.1.(a)XRD pattern of the as-prepared ZnO products.(b)Low-magni?cation.

(c)High-magni?cation FESEM images of the ZnO products.

5g(～1.05M),which take on bowknot-like morphologies.These bowknots are built from several to tens of bipyramidal ZnO twinned crystals with average diameters of810nm and lengths of10–20?m.The inset?gure clearly shows the details of an individual bowknot-like microcrystalline.When10g Na2CO3

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Physics 106 (2007) 58–62

Fig.2.(a and b)TEM images of the as-prepared ZnO products,indicating the coexistence of wire-like and belt-like nanostructure.(c)The TEM image of an individual ZnO nanobelt and corresponding SAED pattern (inserted at the upper left corner).(d)HRTEM image taken from an individual ZnO nanowire.

(～2.10M)was added into the reaction system,a great deal of dumbbell-like ZnO twinned crystals with average diameters of 550nm and lengths of 8–12?m were obtained (Fig.3(b)).In addition,another novel structure coexisting with them—hollow hexagonal prism with one closed end (see the enlarged part inserted in the lower right of Fig.3(b))was observed.Most of them assembled to forming ?ower-like aggregates.It is sug-gested that the growth mechanism for this novel structure should be similar to that for the reported Te nanotubes [22,23].In this case,secondary nucleation and growth would preferen-tially occur at the circumferential edges of the hexagonal prism because these sites had relatively higher free energies than other sites on its surface.After a period of rapid growth,the concen-tration of Zn(OH)42?was decreased,which could not satisfy the growth of rod.Thus,the hollow prism would be formed because of no mass transportation to the inner region.When the adding amount of Na 2CO 3was increased to 15g (～3.54M),some wire-like ZnO nanocrystals with average diameters of 180nm and lengths up to 15?m appeared in the synthesized products (Fig.3(c)).Large quantities of ZnO nanowires with average diameters of 80nm and lengths up to 20?m were produced when the adding amount of Na 2CO 3was 20g (～4.72M),and only

H.Hu et al./Materials Chemistry and Physics 106 (2007) 58–62

Fig.3.SEM or FESEM images of ZnO products prepared from different adding amount of NaCO3under hydrothermal conditions in the absence of SDSN:(a)5g;

(b)10g;(c)15g;(d)20g.

several belt-like nanocrystals were also occasionally observed (Fig.3(d)).The above comparison experiments suggested that the length/diameter of1D structured ZnO strongly depend on the amount of Na2CO3.When the solution changed from undersatu-ration(5g Na2CO3),lower saturation(10g Na2CO3),saturation (15g Na2CO3)to high supersaturation(20g Na2CO3),the diam-eter of1D ZnO crystallites was greatly reduced from micro to nanosize.

Generally speaking,the morphology control of ZnO under hydrothermal conditions should be determined by two main factors:one is the internal crystal structure of ZnO,the other is the selected external condition,such as reaction temperature, reaction additive,mineralizer and so on.It is well known that ZnO is a polar hexagonal and highly anisotropic crystal,and its oriented growth direction is along the c-axis[24].From a kinetics point of view of ZnO crystal growth,Zn(OH)42?is proposed to be the growth unit and is directly incorporated into ZnO crystal lattice at the interface under given conditions[24]. In the process of operating comparison experiment,due to the different amount of mineralizer Na2CO3,the concentration of Zn(OH)42?is also different in the aqueous solution,which will result in different nucleation(such as nuclei size)and crystal growth and further affect the ratio of length/diameter of1D ZnO crystallites.

Based on the above analysis,we proposed that a higher super-saturation solution is the key driving force for the growth of ZnO nanowires.A possible formation mechanism of the ZnO nanowires may be described as follows:?rst,an supersaturate Zn(OH)42?precursor solution is obtained under appropriately higher concentration of Na2CO3(20g).Then,in the initial stage of hydrothermal decomposition,much smaller nuclei are pro-duced through a short burst of homogeneous nucleation process [25].Moreover,the size of a nucleus will determine the lateral dimension of an1D ZnO nanostructure.Finally,after the nucle-ation step,the growth units of Zn(OH)42?are subsequently incorporated into these seeds along the c-axis of ZnO crystal lattice[24].With the prolongation of reaction time,ultra-long ZnO nanowires can be fabricated.

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Fig.4.Photoluminescence spectrum of ZnO nanowires and nanobelts measured at room temperature.

With respect to the formation of ZnO nanobelts,the direct-ing role of surfactant SDSN is undoubtedly signi?cant.In the absence of SDSN,the obtained ZnO products were mainly com-posed of nanowires,as well as occasionally several nanobelts. We proposed that the possible function of SDSN is to kineti-cally control the growth rates of different crystalline faces of ZnO crystals by interacting with these faces through adsorp-tion and desorption under suitable kinetic growth condition,and ?nally resulting in the formation of ZnO nanobelts.The exact role of SDSN in the present system is still under investigation.

The room temperature PL spectrum of as-prepared ZnO nanowires and nanobelts,shown in Fig.4,was obtained with an excitation wavelength of325nm.Two luminescence bands, including a weak UV emission centered at379nm and a strong broad yellow emission with peak located at564nm,were observed.The UV emission band was attributed to the near-band emission of the ZnO products,coming from the direct recom-bination of the conduction band electrons and the valence band holes.The deep-level involved in the yellow luminescence was attributed to interstitial oxygen[26–29].

4.Conclusion

In summary,ZnO nanowires(～60%)and nanobelts(～40%) have been synthesized on a large scale via a low temperature one-pot hydrothermal technique.The experimental results reveal that mineralizer Na2CO3was introduced not only as alkaline source but also as a controllable reagent for the crystal growth of ZnO.The adding amount of Na2CO3could affect the concentra-tion of Zn(OH)42?precursor,and made the average diameters of1D ZnO decrease from810to80nm with the increase of Na2CO3.In addition,the surfactant SDSN played an assisting role in controlling the growth of belt-like ZnO.It is proposed that the possible function of SDSN is to kinetically control the growth rates of different crystalline faces of ZnO crystals by interacting with these faces through adsorption and desorption under suitable kinetic growth condition,and?nally resulting in the formation of ZnO nanobelts.The present strategy of fabri-cating ZnO nanowires/belts is simple,reproducible,high yield, easily operating and may be applied to scale up to industrial production.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(Grant No.20501002),the Education Department of Anhui Province(Grant No.2005KJ110),and Anhui Provincial Young Teacher Sustentation Project of Anhui (Grant No.2005jq1147zd).

References

[1]W.U.Huynh,J.J.Dittmer,A.P.Alivisatos,Science295(2002)2425.

[2]X.F.Duan,Y.Huang,Y.Cui,J.F.Wang,C.M.Lieber,Nature409(2001)

66.

[3]S.Frank,P.Poncharal,Z.L.Wang,W.A.de Heer,Science280(1998)

1744.

[4]H.M.Huang,S.Mao,H.Feick,H.Q.Yan,Y.Y.Wu,H.Kind,E.Weber,

Russo,P.D.Yang,Science292(2001)1897.

[5]V.A.L.Roy,A.B.Djurisic,W.K.Chan,J.Cao,H.F.Lui,C.Surya,Appl.

Phys.Lett.83(2003)141.

[6]S.Liang,H.Sheng,Y.Liu,Z.Huo,Y.Lu,H.Shen,J.Crystal.Growth225

(2001)110.

[7]N.Saito,H.Haneda,T.Sekiguchi,N.Ohashi,I.Sakaguchi,K.Koumoto,

Adv.Mater.14(2002)418.

[8]K.S.Weissenrieder,J.Muller,Thin Solid Film300(1997)30.

[9]N.Golego,S.A.Studenikin,M.J.Cocivera,Electrochem.Soc.147(2000)

1592.

[10]J.Y.Lee,Y.S.Choi,J.H.Kim,M.O.Park,S.Im,Thin Solid Films403

(2002)553.

[11]W.C.Shih,M.S.Wu,J.Crystal.Growth137(1994)319.

[12]Z.W.Pan,Z.R.Dai,Z.L.Wang,Science291(2001)1947.

[13]X.Y.Kong,Z.L.Wang,NanoLetters3(2003)1625.

[14]X.Y.Kong,Y.Ding,R.Yang,Z.L.Wang,Science303(2004)1348.

[15]J.Q.Hu,Q.Li,X.M.Meng,C.S.Lee,S.T.LeeChem,Materials15(2003)

305.

[16]X.H.Kong,X.M.Sun,X.L.Li,Y.D.Li,Mater.Chem.Phys.82(2003)997.

[17]W.I.Park,D.H.Kim,S.W.Juang,G.C.Yi,Appl.Phys.Lett.80(2002)

4232.

[18]X.H.Zhang,S.Y.Xie,Z.Y.Jiang,X.Zhang,Z.Q.Tian,Z.X.Xie,R.B.

Huang,L.S.Zheng,J.Phys.Chem.B107(2003)10114.

[19]J.J.Wu,S.C.Liu,C.T.Wu,K.H.Chen,L.C.Chen,Appl.Phys.Lett.81

(2002)1312.

[20]M.S.Mo,J.M.Yu,L.Z.Zhang,S.A.Li,Adv.Mater.17(2005)756.

[21]A.Wei,X.W.Sun,C.X.Xu,Z.L.Dong,Y.Yang,S.T.Tan,W.Huang,

Nanotechnology17(2006)1740.

[22]B.Mayers,Y.N.Xia,Adv.Mater.14(2002)279.

[23]Z.P.Liu,S.Li,Y.Yang,Z.K.Hu,S.Peng,J.B.Liang,Y.T.Qian,New J.

Commun.27(2003)1748.

[24]W.J.Li,E.W.Shi,W.Z.Zhong,Z.W.Yin,J.Crystal Growth203(1999)

186.

[25]C.B.Murray,D.J.Norris,M.G.Bawendi,J.Am.Chem.Soc.115(1993)

8706.

[26]D.Li,Y.H.Leung,A.B.Djurisic,Z.T.Liu,M.H.Xie,S.L.Shi,S.J.Xu,

W.K.Chan,Appl.Phys.Lett.85(2004)1601.

[27]X.L.Wu,G.G.Siu,C.L.Fu,H.C.Ong,Appl.Phys.Lett.78(2001)2285.

[28]L.E.Greene,https://www.360docs.net/doc/ce7458596.html,w,J.Goldberger,F.Kim,J.C.Johnson,Y.Zhang,R.J.

Saykally,P.Yang,Angew.Chem.Int.Ed.42(2003)3031.

[29]C.C.Lin,S.Y.Chen,S.Y.Cheng,J.Crystal.Growth283(2005)141.